1. Field of the Invention
The invention relates to a voltage regulator, and more particularly to a voltage regulator of an AC-DC converter.
2. Description of the Related Art
In general, AC-DC converters are used to operate directly from an alternating current (AC) input line. In an AC-DC converter, a rectifier circuit may directly convert an AC input voltage from the AC line to a direct current (DC) voltage with ripples. A voltage regulator disposed in an output terminal of the rectifier circuit may regulate the DC voltage with ripples to reduce the ripples.
FIG. 1 shows a conventional AC-DC converter 100. The AC-DC converter 100 comprises a rectifier circuit 110 and a shunt regulator circuit 120. The rectifier circuit 110 converts a high AC voltage HVAC to a high DC voltage HVDC (single polarity voltage). The DC voltage HVDC is not a constant voltage due to the ripples thereof. In order to provide a steady DC voltage HVDC, a smoothing capacitor C1 is used to filter the DC voltage HVDC, so as to reduce the ripples. The regulator circuit 120 comprising a resistor R, a zener diode Z and a capacitor C2 may remove any remaining ripples, and maintain the regulation for various supply voltages and loads. The resistor R and the zener diode Z generate a regulated DC voltage according to the DC voltage HVDC. The capacitor C2 is connected across the zener diode Z to further reduce ripples and diode noise.
The rectifier circuit 110 and the shunt regulator circuit 120 are formed with discrete resistors, capacitors and diodes. However, discrete components increase costs and required printed circuit board area. Furthermore, due to a continuous current flowing through the resistor R and the zener diode Z, in the regulator circuit 120, power is continuously dissipated and loss. Meanwhile, if a conventional regulator circuit is supplied with a high voltage (such as 120V or 240V AC), power dissipation will worsen. Therefore, an ultra high voltage regulator is desired.